Messages with severity="DEBUG" are not output through the Web API

The [code for `px:message`](https://github.com/daisy/pipeline-modules-common/blob/master/common-utils/src/main/java/org/daisy/common/xproc/calabash/steps/Message.java) is essentially this:

``` java
public void run() throws SaxonApiException {
String severity = getOption(_severity, "INFO");
String message = getOption(_message, (String)null);
if ("WARN".equals(severity)) {
warning(this.step.getNode(), message);
} else if ("DEBUG".equals(severity)) {
// DefaultStep#fine() removed in ndw/xmlcalabash1@ce9b07d
this.runtime.getMessageListener().fine(this,this.step.getNode(), message);
} else {
info(this.step.getNode(), message);
}
```

The DEBUG messages are for some reason not included in the job XMLs `` section.

@capitancambio maybe you have an idea why this is? They appear in the detailed log.